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Full text of "Properties of water-repellent fabrics"

U. S. Department of Commerce 
National Bureau of Standards 



Research Paper RP1762 
Volume 38, January 1947 



Part of the Journal of Research of the National Bureau of Standards 



Properties of Water-Repellent Fabrics 



By John W. Rowen and Domenick Gagliardi 



A review and an analysis of the theory of water repellency of textile fabrics have been 
made. The physicochemical basis underlying the wettability, or water repellency, of 
treated fabrics is discussed. A survey of the laboratory test methods for evaluating water 
repellency of textile fabrics is presented. A study was made of 'the water-repellent proper- 
ties of 11 commercial raincoat and 4 military fabrics. For this study two of the more 
recent test methods were examined, the drop-penetration and the contact-angle tests. 
Two other, and older, test methods were also studied, the spray-rating and the hydrostatic- 
pressure tests. Several exploratory observations were made in an attempt to determine the 
mechanism by which water-repellent fabrics lose their repellency when exposed to rain. 



I. Introduction 

The National Bureau of Standards has been 
called upon from time to time to supply informa- 
tion dealing with "water-repellent" fabrics. These 
requests have usually come from Government 
agencies and the public at large. In recent 
months there has been an intensified interest in 
this subject, and the Bureau decided to review 
the field of "water repellency" and to carry out 
a laboratory study in order to answer present and 
future questions regarding this subject. 

An examination of the literature [1] 1 revealed 
the existence of a large number of chemical com- 
pounds and commercial treatments about which 
extraordinary claims were made. Further inves- 
tigation showed that a large number of laboratory 
test methods had been devised for evaluating 
"water repellency." Moreover, considerable con- 
fusion was encountered concerning the interpreta- 
tion of the test results and the meaning of such 
terms as "waterproof", "water-repellent", "shower- 
proof", and "water-resistant" fabrics. It appeared 
desirable, therefore, to make a survey of the field 



i Figures in brackets indicate the literature references at the end of this 
paper. 



of "water repellency" and to carry out some 
laboratory studies on currently available materials. 
This report, therefore, will deal mainly with the 
fundamental nature of "repellency", the labora- 
tory methods used to evaluate it in fabrics, and 
some laboratory observations by the authors on a 
variety of raincoat fabrics. 

II. Theory of n Water Repellency' ' 

We must at the very beginning distinguish 
between "waterproof" and "water-repellent" tex- 
tile surfaces. There is still a great tendency to 
mention both terms simultaneously and inter- 
changeably, in spite of the numerous papers [2, 3,4] 
that have been written to call attention to the 
great difference between them. A waterproof 
fabric is one in which the pores, the open spaces 
between the warp and filling yarns and between 
the fibers, are filled with appropriate substances, 
resulting in a fabric having a continuous surface 
and a very low air permeability. A water-repel- 
lent fabric is one in which the fibers are usually 
coated with a "hydrophobic" type of compound, 
and the pores are not filled in the course of the 
treatment. The latter types of fabrics are quite 



Water-Repellent Fabrics 



103 



permeable to air and water vapor. The charac- 
teristics of the two types of fabric surfaces are 
summarized below: 





Waterproof 


Water repellent 


Pores ...... 


Filled 

Very small . ... .. 


Unfilled. 
Small or large. 


Water-vapor perme- 
ability. 






Air permeability 

Chief characteristic.. 


Small 


Usually large. 


Extremely resistant 


Resistant to wetting by 




to passage of water 


rain drops and to the 




even under a hydro- 


spreading of water over 




static head. 


the textile surface, but 
permits the passage of 
water under a hydro- 
static head. 



From the foregoing it is seen that a "water- 
repellent" textile surface may never be truly 
waterproof and that a "waterproof" textile surface 
does not necessarily have to be water repellent. 
Unless the pores of a raincoat or shower-resistant 
jacket are large enough to permit transpiration, 
the wearer will be extremely uncomfortable due 
to excessive perspiration. The water-repellent 
fabrics are, therefore, more suited in this respect 
for making raincoats or shower-resistant jackets. 
The subject of waterproofing of textiles is very 
extensive and beyond the scope of this paper, 
therefore the discussion is restricted to the sub- 
ject of water-repellent textile surfaces. 

As the term implies, a water-repellent textile 
surface is one that appears to "repel" water. In 
such cases, the water does not wet or penetrate 
the surface of the textile fabric. The tendency of 
a solid to resist wetting is a function of the chemi- 
cal nature of the solid surface, the roughness of the 
surface, the porosity of the surface, and the pres- 
ence of other molecules on the surface. When a 
drop of water comes to rest on a solid, it may 
assume one of the forms shown in figure 1, or any 
intermediate form from complete wetting to com- 
plete nonwetting. Which form the drop will take 
is determined by the above-mentioned variables. 

Examination of the angle of contact (fig. 1) — 
the angle formed by the tangent to the drop at the 
point of contact with the surface (angle measured 
through the liquid) — reveals that the size of the 
angle is related to the repellency of the surface. 
When the angle of contact is small (under 90 
degrees) the surface is said to be wettable [5, 6], 
and when the angle is large (over 90 degrees) the 






SOLID 

c 

Figure 1. — Shapes of drops on solid surfaces. 

A, Contact angle'between 90 and 180 degrees; B, contact angle of 90 
C, contact angle between and 90 degrees. 



)SA 




)SL 



Figure 2. — Forces acting at equilibrium on drop-solid system, 

surface is"^said|to be nonwettable, or repellent. 
Further consideration of the energy relationship 
of the drop-surface system as depicted in figure 2 
reveals that the equilibrium condition is due to 



104 



Journal of Research 



the following surface energies: y ia) y taf y 8t) where 
y sa is the free surface energy per square centimeter 
(numerically and dimensionally equivalent to the 
surface tension) of the solid in contact with air, y la 
is the free surface energy per square centimeter of 
the liquid when in contact with air, and y sl is the 
free surface energy per square centimeter of the 
solid when in contact with the liquid. At equilib- 
rium, the following relationship holds: 



7*a=7ta COS + 7, j. 



(1) 



Dupre [7] first derived the equation relating the 
free surface energies with the work of adhesion: 



W= 



-yia—yu- 



From eq 1 and 2 it follows that 

W=y la (l+cos6). 2 



(2) 



(3) 



Therefore, by measuring 6 and the surface tension 
of the liquid, one may calculate the work of 
adhesion between the liquid and the surface of 
the solid. It is clear that the smaller the work of 
adhesion, the smaller will be the wettability and 
the greater will be the repellency. For poorly 

2 The work of adhesion (W) defined above is the amount of energy necessary 

to destroy 1 cm 2 of liquid-solid interface and to form 1 cm 2 of liquid and a 
1 cm2 of solid surface in contact with air. The energy of a unit area of solid 
surface in air may be somewhat different from the energy of a unit area of solid 
surface in vacuum. The difference between these two energies will depend 
upon the relative humidity, the hydrophobicity of the surface, the presence 
of transient gaseous or solids in the air, etc. It is physically impossible to 
separate a solid and a liquid without leaving the solid covered with at least 
a part of a monolayer [6, p. 62; 8]; however, it is desirable to define a work for 
such a process, which is shown to be given by 

W=y l0 +yu— y»u (10 

where y s0 is the free surface energy per square centimeter of solid in vacuum, 
7i e is the free surface energy per square centimeter of liquid in equilibrium 
with its vapor, and y»i has the same meaning as before. Now, for the clean 
solid in vacuum, the following relationship (analogous to eq 1) may be 
postulated: 

7»» = 7i* cos d+y,i, (2') 

where y tv is the free surface energy per square centimeter of the solid in 
equilibrium with saturating vapor of theliquid. Itfollowsfromeql'and2'that 

W'= 7«o-7« .+7 1 *0+C0S 6) .| (3') 

If we assume yi v =yi a (which approximation is almost within experimental 
error) , we have 

W' = y t0 -y,v+W. (4') 

The quantity 7 g0 — y gv may be looked upon as the free energy of immersion, 
at constant temperature, of a unit surface of clean solid immersed in a satu- 
rated vapor and is given by [8] 



y»o-y**=RT\ 



P 

r 2 rf In P, 



(5') 



where nis Gibbs' "surface density" of the vapor on the solid surface and 
equal to q/M2—TJV2, where q is the quantity of water vapor adsorbed per 
gram of absorbent, M is the molecular weight of the vapor, 2 is the area in 
square centimeters per gram of solid, T is the thickness of the surface region, 
and V 2 is the molal volume of the vapor. 

The right-hand member of 5' can be integrated if an equation for the ad- 
sorption isotherm is known or, if not, a graphical integration of the area under 
the isothermal curve can be carried out. 



repellent, easily wettable surfaces, the drop of 
liquid will assume a shape similar to C in figure 1; 
and for highly repellent, nonwettable surfaces, the 
drop will assume shape A. 

As textile fabric surfaces are not smooth, con- 
tinuous surfaces, but rather porous, screen- like 
surfaces, one must examine the above considera- 
tions and see how they apply to textiles. Figure 
3 shows the cross section of an idealized fabric. 




oooooo 



ajaoob 



o 



B' 



Ficure 3. — Drops on surface of idealized yarn. 



In this figure the parallel yarns are shown as 
circles. If the angle of contact between water 
and the surface of the fabric is larger than 90 
degrees, the equilibrium position of the water 
level of a drop of water will be, as indicated by the 
line AA, well outside the fabric. If, however, the 
angle of contact is much smaller than 90 degrees, 
the water will penetrate the pores and the level 
will fall to some position such as BB. Cassie and 
Baxter [9] have shown that in the case of porous 
surfaces the apparent angle of contact is related 
to the continuous surface-water angle of contact 
in the following way: 



where 



cos Ba^jx COS0— / 2 , 



(4) 



a = apparent angle of contact 
0= angle of contact 

/x=the fraction of the plane geometrical area 
of unity parallel to the rough surface 
occupied by the solid-liquid interface. 

-f 2 ~ the fraction of the plane geometrical area 
of unity parallel to the rough surface 
occupied by the liquid air interface. 

They also derived the relationships between the 
"J" variables, the distance between the fibers in 
the yarns, the radius of the fibers, and the angle of 
contact: 



ft 



~r+d 



(-4) 



J-2- 



r+d 



sin 6, 



(5) 



(6) 



Water- Repellent Fabrics 



105 



where 

r= radius of the fibers 
d= one-half the distance between the fibers 
6= contact angle between the water and the 
fiber. 

From the above, it appears as though the most 
water-repellent fabric will be the one on which the 
drop assumes a form as shown in figure 3, A. 
This condition may be obtained by adjusting the 
variables previously mentioned as being responsi- 
ble for the form of the drop. It is advisable to 
consider each of the variables separately. 

Chemical nature of the solid surface. — When puri- 
fied, the natural fibers, cotton, wool, silk, etc., are 
hydrophilic in character and hence the drops 
assume shapes similar to C (fig. 3). It is common 
practice, therefore, to treat fabrics intended to be 
water repellent, with various hydrophobic com- 
pounds — compositions of waxes, petroleum-like 
molecules, and soaps of polyvalent metals, which 
deposit long-chain hydrocarbon molecules on the 
surface of the fabric. When properly treated with 
a water-repellent agent, the surface of the fabric 
will cause a water drop to assume a form very 
similar to A in figure 3. 

Roughness of the surface. — Wenzel [10] pointed 
out that roughness has a peculiar effect on the 
angle of contact. He employed Freundlieh's [11] 
concept of "adhesion tension/' which is defined as 
follows: 

AT=y la cos6. (7) 

It is noted that the adhesion tension is the differ- 
ence between the work of adhesion per square cen- 
timeter (eq 3) and the surface tension. Wenzel 
recognized that eq 7 was true only in the case where 
the surface was a mathematical plane. As most 
surfaces are not of this type, there is associated 
with each surface a roughness factor R, which is 
the ratio of the actual surface to the geometric 
surface. Wenzel showed the validity of the fol- 
lowing equation: 

AT=y la c -°^. (8) 

Porosity of the surface. — Cassie and Baxter [9, 12] 
pursued the idea of the roughness factor and 
showed that the apparent adhesion tension of a 
porous surface was given by 



where the symbols have the same meaning as in 
eq 4. Using a hexagonal idealized yarn pattern 
as a model, the above workers showed that o-, the 
bulk density of the yarn, was related to the radius 
of the fibers in the following way: 



A2V3) 



pr 



(r+d)' 



(10) 



.4r„ = COS 0aYta = 7ta(/l COS 0—/ 2 ), 



(9) 



where p is the density of the fiber. With the aid 
of eq 4, 5, 6, and 10, they were able to obtain 
plots of contact angle versus a. These plots 
showed that the angle of contact approaches 180 
degrees as a approaches small values (about 0.1 
g/ml). Their theory led them to the position that 
fabric structure was extremely important in the 
production of water-repellent fabrics. 

Presence of other molecules on the surface. — It is 
a well-known fact that the surface properties of 
solids may be greatly altered by covering the sur- 
face with impurities, greasy films, dust particles, 
water vapor and gases, etc. The presence of sur- 
face impurities may or may not be related to the 
fact that numerous workers [12, 13, 14, 15, 16, 17] 
have reported not one angle of contact with a 
particular system, but two angles, an advancing 
and receding angle. One angle is obtained when 
water has advanced over a surface and the other 
when water has receded from the surface. This 
difference in contact angle is often called the 
hysteresis of the contact angle. The cause of the 
hysteresis is still not clearly understood. Re- 
cently, Harkins [18, 19], working with hydrophobic 
materials and an improved method of measuring 
contact angles, found no hysteresis. Langumir 
[17] suggests that the hysteresis of contact angles, 
especially with water, is due to the presence of a 
surface layer of molecules with one end hydro- 
philic and the other hydrophobic, which are 
overturned by the receding water. 

From the above considerations of the variables 
involved in water repellency, one would expect 
that a fabric having a high contact angle would 
be a water-repellent fabric. In such cases the 
water level will remain well outside the surface, 
as shown in figure 3A. At this point it looks as 
though we may be able to write a prescription for 
the optimum conditions for water repellency, viz, 
a textile fabric in which each fiber is completely 
coated with the material having the highest solid/ 
water angle of contact. It is unfortunate that 
this prescription has never been tested. The 



106 



Journal of Research 



reason for this is that the present methods of 
treating fabrics for water repellency do not 
guarantee the complete coating of each individual 
fiber. These treatments leave many of the inter- 
twined fibers without any surface coating. The 
presence of hydrophilic surfaces in the treated 
fabric will of course greatly diminish the over-all 
resistance of the fabric to wetting. 

III. Survey of Methods Used for Measuring 
Water Repellency in Fabrics 

The number of test methods devised for meas- 
uring water repellency appear to be almost as 



many as the number of laboratories engaged in 
this field. Comparative data from and references 
to the various reports on laboratory test methods 
have been assembled in table 1. The object in 
most of these tests has been to offer a sensitive 
and accurate method of predicting the relative 
performance of fabrics in the rain. Very few, if 
any, of the published papers, however, have fur- 
nished any data showing the correlation between 
the particular test method and the performance 
of fabrics in the rain. Examination of the liter- 
ature reveals that there is a considerable variation 
in the methods of testing and the interpretation 
of the results. It was found convenient for the 



Table 1. — Methods and testing conditions used by various laboratories in measuring water-repellent properties of textile fabrics 



Name of test 



Reference 

and year 



Test tempera- 
ture 



Sample size 



Result units 



Extent of use 



CLASS A TEST 



Hydrostatic pressure 

Suter hydrostat ic pressure 

Mullen (high-pressure range) 

Box test for waterproofness 

British (DSIR) 

AATCC spray 

I >n>p penetration 

Water penetration (drip) 

Bundesmann 

Impact penetration 

Spray test 

Kern 

Franz and Henning 

Official German rain 

Spray penetration 

Immersion 

ASTM water-absorption (spray) method 

Dynamic absorption 

Bundesmann 

Becker submersion method 

Esslinger submersion method 

Contact angle by tensiometer 

Wetting test 

Wenzel method 



[21] 


1941 


[22] 


1940 


[22] 


1944 


[24] 


1936 


[25] 


1925 



0°±1° F. 
0°±1° F. 



3"X3". 
4"X4". 



4"X4"„-. 
4HX20",. 

4.8cm2_. 



cm 

cm 

lb/in. 2 

ml/min 

cm 

g,% 

min 

sec. _. 

ml... 

g 

g,% 

No. of drops 

g,% 

g,% 

ml 

gy% 

g,% 

g,% 

g,% 

g,% 

g,% 

deg 

sec 

deg_. 



Wide, ASTM, and AATCC. 
Federal Specification CCC-T-191a and 
Supplement. 

Do. 
Limited. 
Obsolete. 



CLASS B TEST 



[26] 


1941 


[27] 


1944 


[28] 


1943 


[29] 


1935 


[34] 


1943 


[31] 


1937 


[32] 


1933 


[23] 


1936 


[34] 


1910 


[35] 


1946 



80°±1° F 
80°±1° F 



o ±1 o F 



12°±2° C. 



7"X7" 



8"X9"_ 



4"X4"..-. 
154 cm 2... 
6"X8"~- 
6"X6"-.._ 

4cm 2 

12X26 cm. 
28X38 cm. 
12"X12"__ 



Very wide; AATCC; Federal Specifica- 
tion CCC-T-l91a and Supplement. 

Federal Specification CCC-T-191a and 
Supplement. 

ASTM. 

British Specifications; Germany. 

Limited. 
Do. 

Germany. 

Do. 
U. S. Navy. 



CLASS C TEST 



[36] 


1941 


[37] 


1943 


[38] 


1944 


[39] 


1935 


[40] 


1935 


[41] 


1940 



0°±1° F. 



80°±1° F..._ 
Room temp. 



3"X3" 

8"X8" 



8"X8"-.-. 

154 cm 2... 

. r >X7. r >cm 
15X 5 cm. 



Wide; AATCC. 

Federal Specification CCC-T-191a and 
Supplement. 
Do. 
British Specifications; Germany. 
Germany. 
Do. 



CLASS D TEST 



[42] 


1945 


[43] 


1945 


[44] 


1936 



35° C_ 



0.196 in. 2.... 
Single yarn. 
1"X3" 



Experimental. 
Do. 
Do. 



1 Interrogation mark signifies temperature not indicated. 

Water-Repellent Fabrics 



107 



purpose of this report to arrange the different 
test methods in four general classes: 

Class A. Methods by which the hydrostatic 
pressure required to force water 
through a fabric is measured. 

Class B. Methods by which surface wetting and 
penetration under the influence of 
falling drops is measured. 

Class C. Methods by which the absorption of 
water by the fabric, when immersed 
or manipulated under water, is 
measured. 

Class D. Methods by which the wettability of 
the surface of the fabric is measured 
by means of the angle of contact or 
some function of the angle of contact. 

Some timely comments about the more popular 
test methods in each group are given below. 

Class A. — In this class of test methods, the 
fabric is subjected to the action of water under 
pressure by a variety of means. Either the 
amount of water penetrating in a specified time 
or the pressure required to force water through the 
fabric is measured. The most widely used test 
methods in this class are the AATCC [21] and the 
Suter [22] hydrostatic-pressure tests. Both meth- 
ods give reproducible test values. It has been 
reported [45] that a correlation exists between the 
two methods. The following relationship is sup- 
posed to hold at 27° ±3° C: 



^=d.32S-4, 



(11) 



where A is the AATCC test value, S is the Suter 
test value. 

The hydrostatic-pressure test values are de- 
pendent mainly on the pore size and the angle of 
contact. The bulge that occurs when the pres- 
sure is applied on the fabric requires a correction 
[461, but in view of the reproducibility of the test 
and the smallness of the correction factor, the 
extra measurements and calculations are usually 
avoided. 

Class B. — The tests in this class are in some 
cases very different from each other. They are 
all, moreover, quite different from those in class 
A. All the tests in this class subject the test 
fabric to the action of water drops. The num- 
ber, size, frequency, and energy of the drops in 
the various tests vary considerably. The ad- 



vantages and disadvantages of these various 
tests are briefly discussed below: 

1 . AATCC Spray Test. This test is very widely 
used in the textile industry for control work. The 
test will distinguish qualitatively between treated 
and untreated fabrics. It is not able, however, 
to distinguish between obviously different water- 
repellent finishes or to predict the performance of 
a fabric in the rain. 

2. Drop-Penetration Test. — This test was 
developed for and used by the United States 
Quartermaster Corps during the war. The energy 
of the drops used in this apparatus is approxi- 
mately 10,000 to 15,000 ergs. This value is 
much larger than that of the drops in a cloud- 
burst (3,000 ergs). It appears that by this test 
one may be able to arrange fabrics in what seems 
to be a proper order of protection in the rain. 
The hardness of the fabric backing has a pro- 
found influence on the test value. This is prob- 
ably also true for the other tests in this group. 
It should be mentioned that the values obtained 
in the drop penetration test may differ as much as 
20 percent among each other, depending on the 
particular fabric and its test value. 

3. Bundesmann Test. — The Bundesmann test 
was developed in Germany. It can be used in 
two ways: (1) to measure the amount of water 
penetrating the sample, or (2) to measure the 
amount of water absorbed by the sample in a given 
time. Again the drops of water in the test have 
energy of from 15,000 to 30,000 ergs, which is a 
much larger value than is found in a cloudburst. 
In this procedure the sample of cloth is in constant 
motion and is continuously rubbed on the under 
side during the test. When the amount of water 
absorbed is measured, the mean deviation [33] of 
the test value is reported to be about 3 percent. 
The method seems to offer some advantages over 
the other tests in class B and has been recently 
adopted by the British Sub-Committee of the 
General Technical Committee as a standard test 
method. 

4. Impact Penetration Test. — This test in- 
volves the spraying of 500 ml of water on the test 
specimen. At the end of the spray period, a 
blotter beneath the fabric is weighed. The 
increase in weight of the blotter represents the 
amount of water that passed through the cloth. 
No data are available on correlation with natural 
rain. 



108 



Journal of Research 



5. Spray Test. — No temperature control is 
specified in this test. It appears to be similar to 
the impact penetration test. No data are avail- 
able for correlating the results of the test with 
results obtained with fabrics in natural rain. 

6. Kern Test. — This novel method has been 
patented. It employs a single drop of water of 
definite size. An electric circuit is used to detect 
the penetration of the water to the under side of the 
test specimen. The average deviation of the test 
value from the mean appears to be smaller for 
smaller size drops. Although there appears to be 
a tremendous variation in the results, depending 
upon the fabric, the author states that seven tests 
are sufficient for obtaining " exact results." 

Class C. — All methods in this group measure the 
resistance that the finish offers to wetting by 
water. As droplet penetration is not used in these 
methods, the part played by the structure of the 
fabric is less important than in other tests in 
determining the test value. The water absorbed 
by the fabric is measured by weighing the test 
specimen after some form of partial (hying. In 
some of the tests (dynamic absorption, Becker, 
etc.), the sample is in motion during the test 
period of exposure to the water. In almost all 
cases the deviation of any particular test value 
from the mean appears to become smaller as the 
exposure time gets larger. The dynamic types of 
tests are definitely to be preferred to the static 
types in this category. Again, it should be pointed 
out that little, if any, work has been done to corre- 
late these test results with the performance of 
fabrics in the rain. 

Class D. — Workers who have used the following 
three technics have done so in an attempt to 
understand the mechanism of wetting. Various 
methods for measuring contact angles [47] have 
been described. The three methods which have 
been applied to fabrics are discussed below. 

1. Wenzel Method. — Wenzel appears to have 
been the first to attempt to apply methods of 
measuring contact angles to fabrics [10]. He 
modified the til ting-plate method [47] to make it 
applicable to fabrics. Wenzel measured the con- 
tact angle of many materials, waxes, cellulose 
acetate, many metal stearates, rosin, and others. 
His data indicated that the method was applicable 
to textile surfaces and was very reproducible. At 
the Bureau it was found possible to reproduce the 
values that Wenzel obtained on paraffin wax on 



glass. It was found, however, that this method 
lacked control over the following variables: (a) 
Temperature, (b) rate of flow of the water over 
the solid surface, (c) rate of rotation of the plate 
in the water, and (d) cleanliness of the surface 
of the liquid. 

2. Wetting Test. — This test measures the 
time for the contact angle on a yarn or strip of 
fabric to decrease to 90 degrees. This time has 
been called the " wetting time." Cassie and Baxter 
claim that the wetting test is more sensitive to 
proofing efficiency than either the Bundesmann 
or the hydrostatic-pressure tests. 

3. Contact Angle by the Tensiometer 
Method. — Wakeham and Skau [42] found that, 
by modifying an ordinary interfacial tensiometer 
and forcing a circular piece of fabric through the 
surface of the water, they obtained a relationship 
between the pressure necessary to force the disk 
of cloth through the surface of the water and the 
angle of contact of the water to fabric. The 
method does not appear to be capable of measur- 
ing angles greater than 120 degrees. Unfortun- 
ately, the upper portion of the measurable range 
is the most useful from the point of view of water 
repellency. 

The great dependence of all test values on the 
temperature of the water has been mentioned in 
several reports [43, 45, 33, 48]. It is quite sur- 
prising, therefore, that many of the methods listed 
in table 1 appeared to be uncontrolled in tem- 
perature. 

The above review of the methods used to 
measure water repellency indicates that no one 
method completely measures the phenomenon. 
It appears necessary that a combination of tests 
be used to evaluate the water repellency of a 
fabric. It appears possible, also, to select one 
test from each class and report the results in the 
same units. For example, the hydrostatic pres- 
sure is increased at the rate of 1.0 cm a second in 
the AATCC test. The test value could be ex- 
pressed in time units. This could also be done 
with the drop penetration, the dynamic absorp- 
tion, the wetting and other tests. One could then 
obtain a summary test value which would be the 
results of n number of test values, all expressed in 
time units: 



T=f (a, 6, c, 



n). 



(12) 



It seems reasonable to assume that the value de- 



Water-Repellent Fabrics 



109 



rived from n number of tests would be a more 
reliable measure of the repellency of a fabric than 
taking the value of only one test. It would re- 
main to be proved, however, whether such a sum- 
mary value gives an accurate and reliable indica- 
tion of the performance of a fabric in the rain. 

Considerable information obtained during the 
war indicates that many of the test methods 
given in table 1 are not applicable for evaluating 
the repellency of woolen fabrics [49]. All the 
test methods mentioned require further investi- 
gation to see whether they can be adapted for 
testing all types of water-repellent fabrics. It is 
also necessary that some of these methods be 
studied for the purpose of setting up commercial 
and Government standard methods of testing. 

IV. Evaluation of Water Repellency by 
Typical Test Methods 

In planning the laboratory work, it was neces- 
sary to take into account the questions that are 
asked by Government agencies, industry, and the 
public at large. The following general questions 
are frequently asked : 

To what degree is a particular fabric water 
repellent? 



To what extent does dry cleaning and/or 
laundering diminish the water repellency of a 
fabric? 

What causes a fabric to lose its repellency dur- 
ing the course of exposure to the rain? 
In order to have available a background of infor- 
mation and data about water-repellent fabrics 
and common test methods, 11 typical commercial 
raincoat fabrics and 4 Army fabrics were selected 
for study. The characteristics of these 15 water- 
repellent fabrics are given in table 2. The water- 
repellent properties were examined by the follow- 
ing test methods: AATCC hydrostatic-pressure 
test, AATCC spray test, drop-penetration test, 
and angle of contact test (tensiometer type). 

Figure 4 shows the results obtained by the four 
test methods. Each test was done according to 
the directions prescribed in Government specifica- 
tions or published papers. It is noted that the 
most widely used test method, the spray-rating 
test, did not show any differences among 12 of the 
15 fabrics. Great differences among the various 
fabrics are revealed by the remaining three tests. 
As each test measures different properties of the 
fabrics (see section on survey of methods, p. 107), 
it is not surprising that the 3 tests do not rank the 



Table 2.— Properties of 15 water-repellent fabrics 



Sample No. 



Type of weave 



Number of plies 



Warp 



Filling 



Thread count 



Warp 



Filling 



Air permeabil- 
ity at a pres- 
sure of 0.5 in. 

of water across 
fabric 



Weight 



WATER-REPELLENT COMMERCIAL FABRICS 



Twill-tackle.. 

do 

Plain-poplin _ 

do 

Twill 



Plain-poplin 

.....do 

Twill-gabardine _ 

Plain-poplin 

Twill 



Plain -poplin. 



1 


3 


125 


65 


1 


3 


125 


67 


2 


1 


107 


51 


2 


1 


109 


52 


2 


2 


135 


58 


2 


1 


107 


55 


2 


1 


107 


54 


2 


1 


127 


54 


2 


1 


106 


54 


1 


1 


104 


60 


2 


1 


107 


56 



{ft 3/min)/ft 2 
20.4 
17.9 

5.0 

6.2 

6.4 

8.0 

5.0 
16.2 

4.6 
23.6 

5.3 



Ozlyd 2 
8.6 
8.9 
5.8 
5.9 
6.4 

5.7 
6.0 
6.5 
6.3 
7.3 



ARMY WATER-REPELLENT FABRICS 



Sateen. 
Oxford. 
Sateen . 
Poplin. 



2 


2 


115 


70 


1 


1 


134 


53 


2 


2 


115 


67 


1 


1 


53 


52 



3.9 
3.2 
3.8 
4.1 



9.1 

6.2 
8.9 
6.2 



110 



Journal of Research 



15 fabrics in exactly the same order. It is noted, 
however, that all fabrics can be approximately 
fitted into 3 general groups: good, fair, and poor. 



100 

90 
o 
z 80 

| 70 

» 60 

2 50 

Q. 

<n 40 

30 

20 

10 





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COMME 


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COMMERCIAL 



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ARMY 








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IMC 


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7391146 2 10 81 5 



COMMERCIAL 



15 24 12 26 7 3 9 10 II 4 8 6 I 2 5 

Figure 4. — Comparison of four water-repellency tests. 



Certain samples always appear in a specific group: 
e. g. No. 5, poor; No. 3, good; No. 6, fair, etc. 
The fallacy of using only one test method to 
evaluate water rcpellency can be clearly seen from 
the data presented. It would be quite impossible 
to classify a particular water-repellent fabric on 
the basis of only one test. 

Both the hydrostatic-pressure test and the 
measurements on contact angles reveal certain 
differences among the various samples not re- 
vealed by the spray test. The drop-penetration 
test, however, appears to be able to show differ- 
ences in the fabrics that were not revealed by any 
of the other tests. The limitations of a par- 
ticular laboratory test for evaluating water re- 
pellence are generally recognized. The most 
frequent argument against a certain test is that it 
does not test the fabric under conditions of actual 
use. Tests that simulate natural rain, therefore, 
are considered much more useful for practical 
testing. The drop-penetration test appears to 
be very useful in predicting the relative perform- 
ance of fabrics in the rain. It seemed desirable, 
therefore, to study the present group of fabrics in 
greater detail on the basis of this test. In view 
of the arbitrariness of the test value — -the time 
to collect 10 ml of water passing through the test 
specimen — it was decided to study the rate of 
penetration of water through water-repellent 
fabrics for long periods of exposure. It was found 
that the "rate of penetration (ml/min) versus 
time curve" was very characteristic of the nature 
of the particular fabric. Figures 5 and 6 show 
plots of the rate of penetration of water against 
time of exposure, for 11 of the fabrics studied. 
The arrows in figures 5 and 6 indicate the time 
at which 10 ml of water had passed through the 
fabric. Each rate curve contains the characteris- 
tic portions: an induction period, an S-shaped 
portion of a curve, and a limiting rate region. 
It is believed that the penetration of water through 
a fabric in natural rain follows a similar pattern 
and that the curves obtained for the drop-penetra- 
tion apparatus are not a characteristic of the 
apparatus alone. These curves reveal that during 
the induction period little or no water comes 
through the cloth. When the resistance to pene- 
tration is finally broken, water passes through the 
fabric at a very rapid rate (S-shaped region). 
Finally, the amount of water that penetrates the 
fabric per unit time reaches a steady state. As 



Water- Repellent Fabrics 

724217—47 8 



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14 15 16 17 18 19 20 



01 234 56789 JO II 12 

TIME IN MINUTES 

Figure 5.— Rates of penetration of water through fabrics in the drop-penetration apparatus. 
Arrows show the point at which 10 ml of water-test value— have passed through the fahrie. 



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TIME IN MINUTES 

Figure 6. — Rates of penetration of water through fabrics in the drop-penetration apparatus. 
Arrows show the points at which 10 ml of water— test value— have passed through the fabric 



20 



112 



Journal of Research 



most of the water-repellent fabrics tested are at 
best only somewhat resistant to penetration, it is 
clear that, after prolonged exposure of a water- 
repellent garment to the rain, the wearer would 
not keep dry. The curves also indicate that the 
longer the induction period, the lower will be the 
equilibrium rate of penetration of water. In 
designing new T water-repellent fabrics, therefore, 
the object would be to fix the various variables 
involved so that the resulting product would have 
a long induction period and a low rate of pene- 
tration. Such a fabric has been designed. The 
Shirley type of cloths that the Army Quarter- 
master Corps developed during the war have a 
very long induction period (several hours) and a 
very low equilibrium rate of penetration (approxi- 
mately 0.1 ml/miii). These types of fabrics are 
highly effective for prolonged use in tbe rain. 

The above studies of water repellency lead to 
the conclusion that obtaining the rate of penetra- 
tion of water with the drop-penetration apparatus 
gives a more complete picture of the repellency of 
a fabric than the 10-ml test value by itself. This 
is especially significant when one is considering 
commercial fabrics (compare fig. 5 with 6). It 
is also concluded that better evaluation of water 
repellency may be made when a group of tests is 
employed. A study of the four test methods 
given above reveals that, as the rank correlation 
among various tests is very low, it is necessary to 
correlate all tests with the behavior of fabrics in 
natural or artificial rain. The Philadelphia Quar- 
termaster Depot has developed and constructed 



[50] a rain room for fabric and garment testing. 
By means of specially designed nozzles, it is 
possible to obtain rainfalls at the rate of 0.1, 1.0, 
and 3.0 inches an hour, having natural and re- 
producible drop-size distributions. 

V. Durability of Water-Repellent Finishes 

The durability of water-repellent finishes is 
generally taken to be the resistance that the 
finish shows to the action of dry cleaning or 
laundering. Durable water-repellent fabrics are 
those whose water repellency is only slightly 
diminished by dry cleaning or laundering. Those 
fabrics whose water repellency is totally or greatly 
impaired by dry cleaning or laundering are classi- 
fied as being nondurable. 1 1 should he remembered 
that the two terms are only relative and that the 
difference between them is a matter of degree of 
resistance and not necessarily of type of compound. 

The effect of dry cleaning and laundering on 
water repellency was studied, using only the 11 
commercial raincoat fabrics given in table 2. 
Standard methods of dry cleaning and laundering 
were used [51]. The results of these studies are 
given in table 3. By the use of three different 
test methods, it was found possible to distinguish 
between durable and nondurable repellent finishes. 
Samples 4, 7, 9, and 11 show a smaller decrease 
in all test values than do the remaining samples 
which reveal fairly large changes in all test values. 
It was found that samples whose water repellency 
had been totally lost by dry cleaning or laundering, 



Table 3. — Effect of dry cleaning and laundering on the water repellency of 11 commercial fabrics 



Sample No. 


spray rating 


Hydrostatic pressure (centimeters 
of water) 


Drop-penetration time (time, in 
minutes, to collect 10 ml of water) 


Control 


Dry 

cleaned 

three times 


Laundered 
three times 


Control 


Dry 

cleaned 

three times 


Laundered 
three times 


Control 


Dry 

cleaned 

three times 


Laundered 

three times 


1 


100 
100 

90 
100 

80 

100 
100 
100 
100 
90 

100 


50 

50 
70 
70 


50 
70 
70 
70 


70 


50 



70 




80 

o 

50 


70 


28.9 
33.0 
40.1 
35.9 
16.5 

33.1 
48.0 
29.5 
37.8 
30.5 

36.3 


36.3 
29.9 
18.4 
33.1 
2.7 

15.7 
40.1 
23.6 
32.4 
5.3 

31.6 


4.0 
3.3 
2.9 
9.0 


2.5 

28.2 


14.9 

2.8 

31.6 


17 
1.5 
6.8 
4.2 
2.1 

2.4 
3.2 
2.0 
3.9 
1.6 

3.9 


1.6 
1.4 
( a ) 
2.3 

(*) 

( a ) 
3.1 
1.1 
2.7 
0.6 

3.0 


1.4 
( a ) 
( a ) 
2.7 
( a ) 

(•) 

3.4 

( a ) 
3.2 
fa) 

4.4 


2 


3.. 


4 

5 


6 


7.. 

8 


9 


10 

11 





a Drop-penetration time could not be measured as the swatches became completely saturated with water at the start of the test. 

Water-Repellent Fabrics 



113 



gave no test value in the drop-penetration test. 
These samples became completely saturated with 
water as soon as they were placed in the test 
apparatus. 

The lowering of the water repellency of fabrics 
by dry cleaning or laundering is commonly 
attributed to leaching of the repellent compound. 
The presence of residual soap on the fabric and 
distortions of the fabric structure are also con- 
sidered responsible for the loss in repellency. It 
has been observed, however, that simple wetting 
by water will also decrease the repellency of a 
fabric. Tests done at the Philadelphia Quarter- 
master Depot [52] have shown that water-repel- 
lent jackets were more easily wetted if they had 
been previously exposed to the rain. It was found 
that drying the jackets after the test exposure, 
even by heating in an oven, failed to restore to the 
garments their original water repellency. In view 
of the interest in the durability of water repel- 
lency, it seemed advisable to attempt to establish 
the rate at which some fabrics lose their repellency 
when exposed to the rain. For this study, four 
swatches each of five different water-repellent 
fabrics were tested five different days in the drop- 
penetration test. At the end of each test period, 
the swatches were allowed to dry at room tem- 
perature and then were conditioned overnight at 




2 3 4 

NUMBER OF TIMES TESTED 



65-percent relative humidity and 70° F. The 
results of this experiment are shown in figure 7. 
It was found that the various fabrics lose their 
repellency at different rates. The drop-penetra- 
tion test values get progressively lower each time 
the swatches are tested. This loss in repellency 
appears to be an irreversible process. Attempts 
to restore the original repellency by heating in 
vacuum at elevated temperatures were unsuccess- 
ful. An analysis of the factors that could be 
causing the loss in repellency leads to the fol- 
lowing: Loss in the repellent agent by leaching, 
creation of new uncoated surfaces due to swelling 
of fibers, and changes in the surface geometry of 
the fabric due to the pounding of the high energy 
drops. Any or all of these changes could be 
taking place, and they all would be irreversible 
changes. 

In order to throw light on the loss in repellency, 
two simple exploratory experiments were made. 
The changes in the contact angle of two of the 
fabrics were measured after various periods of 
wetting in the drop-penetration apparatus. Also, 
the change in contact angle and the increase in 
" standard moisture regain'' 3 of one sample were 
noted after prolonged periods of spraying. The 
spraying was done with the nozzle of the AATCC 
spray test and was continuous for the particular 
period of time. The results of these observations 
are shown in figures 8 and 9. The contact angle 
of the dry fabrics was found to be lower after 



140 








130 








uj 120 








n "0 




T ' ■ • 


24 


1 

UJ 

^ «oo 

z 
< 




t 
10 


r 

§ 80 
u 








70 
60 


" 







8 10 12 14 16 18 

TIME- MINUTES 



Figure 8.— Change in contact angle of fabrics exposed in 
the drop-penetration apparatus. 



Figure 7.— Lowering of the drop-penetration time owing 
to repeated testing. 



3 Regain in weight during a 24-hour exposure period to 65-percent relative 
humidity and 70° F. 



114 



Journal of Research 



each period of wetting in the drop-penetration 
test. The contact angle of one of the fabrics was 
also lowered after spraying the sample for various 
times. Swatches of this fabric, when sprayed 
and then tested for standard moisture regain, 



showed an increase in rate of moisture absorption. 
It is emphasized that the increase is an increase in 
rate (per 24-hour period) as the "repellent" 
fabrics do not always reach equilibrium in this 
length of time. 



i 




CONTACT ANGLE 



6 9 

EXPOSURE TO WATER SPRAY— HOURS 



90 



Figure 9.— Changes in the contact angle and the standard moisture regain of a 9-ounce water-repellent fabric after prolonged 

spraying with water. 



VI. Summary and Conclusions 

The theory of water repellency of textile fabrics 
has been reviewed with special references to the 
more recent theories of the wetting of fabrics by 
water. A survey has also been made on the vari- 
ous testing methods that have been devised 
for measuring water repellency. The results 
of the present investigation lead to the following 
conclusions regarding the status of water repel- 
lency. There is definite need for a comprehensive 
study of the role that the structure of a fabric 
plays in the phenomena of water repellency. In 
the past, the emphasis has been on developing 
more efficient compounds. Data available indi- 
cate that a better understanding of fabric construc- 
tion as it applies to repellency, coupled with the 
now available water-repellent agents, will lead to 
some more nearly idealized type of water-repellent 
garment. In regard to testing methods, it is re- 
quired that correlation be established between the 



results of laboratory test methods and performance 
of fabrics in the rain. 

As already stated, the contact angle is influenced 
by the following factors: The' chemical nature of 
the solid surface, the porosity of the surface, and 
the presence of other molecules on the surface. 
Again, any one or all of these factors could dimin- 
ish the contact angle during wetting of the fabric. 
The change in moisture regain of sample 24 
(fig. 9) shows that the rate at which a fabric 
absorbs moisture increases in proportion to the 
number of times the sample has been wetted. It 
is of interest to examine the surface factors that 
might be responsible for the increase in moisture 
absorption. The loss in repellent agent, the 
change in position of the fibers in the yarns, and 
the creation of new surfaces could all affect the 
rate of water absorption. The swelling of par- 
tially coated or uncoated fibers would also result 
in making available more hydrophilic surfaces 
(OH groups) [53]. 



Water- Repellent Fabrics 



115 



References 

[1] Chemical Abstracts for 1942 and 1943, for example. 
[2] H. P. Pearson, Waterproofing textile fabrics (The 

Chemical Catalog Co., Inc., New York, N. Y., 

1924). 
[3] D. A. Martin, Am. Dyestuff Reptr. 21, 126 (1932). 
[4]~G. A. Slowinske, Am. Dyestuff Reptr. 32, 85 (1943). 
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1399 to 1411 (1930). 
[6] J. Alexander, Colloid chemistry 6, 313 (Reinhold 

Publishing Corporation, New York, N. Y., 1946). 
[7] Dupre, Theorie Mechanique de la Chaleur, 369 

(1869). 
[8] J. Alexander, Colloid chemistry 6, 62 (Reinhold 

Publishing Corporation, New York, N. Y. 1946) ; 

G. E. Boyd, Surface chemistry, p. 133 (Am. Assn. 

Adv. Sci. No. 21, 1943). 
[9] S. Baxter and A. B. D. Cassie, J. Textile Inst. 36, 

167 (1945). 
[10] R. Wenzel, Ind. Eng. Chem. 28, 988 (1936). 
[Ill A. Harvey, Wetting and detergency, p. 18. (The 

Chemical Catalog Co., New York, N. Y., 1937). 
[12] S. Baxter and A. B. D. Cassie, Trans. Faraday Soc. 

40, 546 (1944). 
[13] L. Rayleigh, Phil. Mag. [5], 30, (1890). 
[14] A. Pockels, Phys. Z. 15, 39 (1914). 
[15] F. E. Bartell and Cardwell, J. Am. Chem. Soc. 64, 

164, 494, 1530 (1942). 
[16] J. Ablett, Phil. Mag. [6] 46, 244 (1923). 
[171 N. K. Adam, The physics and chemistry of surfaces, 

p. 181, 2d ed. (Clarendon Press, Oxford, England, 

1938). 
[18] W. D. Harkins, J. Am. Chem. Soc. 62, 3381 (1940). 
[19] J. Alexander, Colloid chemistry 6, 61 (Reinhold 

Publishing Corporation, New York, N. Y., 1946). 
[20] F. L. Gleysteen and V. R. Dietz, J. Research NBS 

35, 285 (1945) RP1674. 
[21] Am. Assn. Textile Chem. Colorists, 1944 Year Book, 

p. 206. 
[22] Am. Assn. Textile Chem. Colorists, 1940 Year Book, 

p. 223. 
[23] U. S. Army Spec. 100-48, p. 20 (May 11, 1945). 
[24] Am. Assn. Textile Chem. Colorists, 1937 Year Book, 

p. 190. 
[25] First report of the fabrics coordinating research com- 
mittee, His Majesty's stationery office, p. 20 

(London 1925). 
[26] Am. Assn. Textile Chem. Colorists, 1944 Year Book, 

p. 210. 
[27] Federal Spec. CCC-T-191a and its supplement 

(Oct. 8, 1945). 
[28] Standards on textile materials, Am. Soc. Testing 

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[29] H. Bundesmann, Melliand Textilber. 16, 128 (1935) 
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[31] A. A. Cook and S. Zaparavik, Am. Dyestuff Reptr. 

26,323 (1937). 
[32] U. S. Patent 2,012,762. 



[33] A. Klingelhofer, H. Mendrzyk, and H. Sommer, Wiss. 

Abhandl. deut, Materialpriifungsanstalt. 30; Chem. 

Zentr. 11, 2249-50 (1940). 
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Abhandl. deut. Materialprufungsanstalt. 23; Chem. 

Zentr. 11, 2249-50 (1940). 
[35] Harry B. Kime, Am. Dyestuff Reptr. 35, 261 (1946). 
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op. 212. 
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(1935). 
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Abhandl. deut. Materialprufungsanstalt. 1, 15; 

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(1945). 
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Reptr. 34, 37 (1945). 
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(Oxford University Press, London, 1941). 
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28, 285 (1939). 
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subsequent personal communication from staff of 

Milton Harris Associates. 
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and 74 to 76d (1944). 
[52] Personal communication from the staff at the Phila- 
delphia Quartermaster Depot. 
[53] K. Lauer, Kolloid-Z. 107, Heft 2, 86 (1944). 

ADDITIONAL REFERENCES NOT CITED IN TEXT 

S. Mierzinski, The waterproofing of fabrics (D. Van 

Nostrand Co., New York, N. Y. 1903); also translation 

of 3d ed. by A. Morris & H. Robson (Scott, Greenwood 

& Co., 1920). 
H. Jaeger, European methods of waterproofing fabrics 

with formulae for ordinary and continuous processing, 

Melliand Textilber. 1, 123 (April 1929); 1, 253 (May 

1929). 
Waterproofing and showerproofing, Textile Colorist 54, 

87, and 130 (1932). 
A. J. Kelley, Textiles: Flame and waterproofing, cotton 

98, No. 12, 53 (1934). 
W. M. Scott, The testing of textiles for waterproofness, 

Am. Dyestuff Reptr. 27, 479 (1938). 
T. Stenzinger, Review and criticism of the methods of 

testing waterproof or water repelling impregnations, 

Am. Dyestuff Reptr. 27, 407 (1938). 



116 



Journal of Research 



R. Flint, The hydrophobing of artificial fibers and fabrics, 

Silk And Rayon 13, 808 (1939). 
C. A. Norris, Water-resistant finishes, Textile Colorist 62, 

165 (1940). 

F. J. Van Antwerpen , Chemical repellents, Ind. Eng. 
Chem. 33, 1514 (1941). 

B. Monsaroff, waterproofing textile fabrics, Can. Chem. 

Process Inds. 26, 265 (1942). 
Eric Croen, Water-repellent and waterproof finishes for 

Textiles, Cotton 1C7, No. 1, 63-66 (1943). 

G. A. Slowinske, Evaluating water repellency of fabrics, 
Textile Mfr. 70, 268 (1944). 

H. P. Holman and T. D. Jarrell, Effects of treating ma- 
terials and outdoor exposure upon water-resistance and 
tensile strength of cotton duck, Textile World 73, 3103 
(1928). 

Effects of waterproofing materials and outdoor exposure 
upon the tensile strength of cotton yarn, Ind. Eng. 
Chem. 15, 236 (1923). 

G. Barr, The determination of waterproofness of "porous" 
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Washington, September 13, 1946. 



Water-Repellent Fabrics 



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